AtCore::initSerial => AtCore::newConnection

Summary:

• AtCore::initSerial => AtCore::newConnection
• AtCore now aware of when machine starts and waits for machine to come up before sending any commands.

Test Plan:

Connect printer with both autodetect and manual plugin loading.

Reviewers: tcanabrava, patrickelectric, laysrodrigues

Reviewed By: tcanabrava, laysrodrigues

Subscribers: yurchor, Atelier: AtCore

Tags: Atelier: AtCore

Differential Revision: https://phabricator.kde.org/D17918